Multilayer articles capable of forming color images

ABSTRACT

Multi-layer articles capable of forming color images are provided. The articles include a multi-layer construction with at least two layers in which at least one of the layers includes a thermally activatable composition. The thermally activatable composition includes a non-linear light to heat converter composition and a color forming compound. Upon activation with a light source an image forms.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage filing under 35 U.S.C. 371 ofPCT/US2009/042650, filed May 4, 2009, which claims priority to U.S.Application No. 61/053,340 filed May 15, 2008, the disclosure of whichis incorporated by reference in its/their entirety herein.

FIELD OF THE DISCLOSURE

The disclosure relates to multi-layer articles capable of forming colorimages and multi-layer articles containing color images.

BACKGROUND

For many applications it is desirable to have articles which containcolor images. A variety of techniques have been developed to form colorimages, including printing techniques and other imaging techniques suchas thermal imaging processes.

Among the thermal imaging processes are “thermal transfer” processes inwhich heat is used to move colored material from a donor sheet to areceiver sheet. Alternatively, heat may be used to convert a colorlesscoating on a sheet into a colored image in a process called “directthermal” imaging. Generally in this process a thermal print head is usedto address one line of the image at a time.

SUMMARY

Multi-layer articles are disclosed which are capable of forming colorimages. Additionally, multi-articles that contain color images are alsodisclosed.

Articles of this disclosure include multi-layer constructions wherein atleast one layer comprises a thermally activatable composition. Thethermally activatable composition comprises a non-linear light to heatconverter composition and a color forming compound.

Also disclosed are articles which include multi-layer constructionscomprising at least two layers, in which at least one layer comprises acolor image and at least one layer comprises a cover layer over thecolor image. The layer that comprises a color image comprises thereaction product of at least one thermally activatable composition. Thethermally activatable composition is capable of being activated by alight source and comprises a non-linear light to heat convertercomposition and a color forming compound. In some embodiments the colorimage comprises more than one layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic cross section of an embodiment of a multi-layerconstruction.

DETAILED DESCRIPTION

In a variety of applications it is desirable to have an article on whichthere is formed a multi-layer color image not on the surface layer orlayers of an article or construction but within internal layers. Thecolor image on such an article is more likely to be more resistant toscratches, stains as well as tampering without requiring lamination ofprotective layers over the image.

Articles containing multi-layer constructions capable of forming colorimages are disclosed. These multilayer constructions contain at leastone thermally activatable layer in which the thermally activatable layermay be activated by a light source. The thermally activatable layercomprises a non-linear light to heat converter composition and a colorforming compound. When the non-linear light to heat convertercomposition absorbs light from a light source, the conversion of lightto heat causes localized heating to occur which induces color formationin the color forming compound. The light source may be chosen to givevery selective activation, for example by using a narrow range ofwavelengths or a single wavelength of light. For example, lasers can beused for this purpose. In some embodiments, the laser may be combinedwith focusing optics to focus the laser light on a specific locationwithin the construction.

Also disclosed are multi-layer articles containing color images. Themulti-layer articles are ones in which the image is not located on theouter surface of the multi-layer construction. Therefore, the image isprotected by an external layer or layers without the need to laminateadditional layers over the image.

The image resides within at least one layer within the multi-layerconstruction. The image comprises the reaction product of a thermallyactivatable composition. The thermally activatable composition comprisesa non-linear light to heat converter composition and a color formingcompound. In some embodiments, the image is a multi-layer image, meaningthat the image resides within multiple layers within the multi-layerconstruction. In such multi-layer images, the multiple layers that makeup the image typically are of different colors. In some embodiments theimage may be a three dimensional image.

The term “multi-layer construction” as used herein refers to aconstruction comprising two or more layers. The layers may be of varyingcompositions and thicknesses. At least one of the layers of themulti-layer construction comprises a thermally activatable composition.

The term “thermally activatable composition” as used herein refers tocompounds and compositions which upon the application of heating orwarming undergo a detectable change in color. The composition may, andtypically does, contain two or more components. The detectable changemay be the formation of a detectable change, such as a change from acolorless or a lightly colored state to a highly colored or a differentcolor state. The term “activation” as used herein refers to the processwhereby a thermally activatable composition absorbs light, the light isconverted to heat by the light to heat converter composition, and theheating causes a detectable color change in the color forming compound.

The term “light to heat converter composition” as used herein refers tocompounds or compositions that generate heat upon the absorption oflight.

The term “non-linear light to heat converter composition” refers to alight to heat converter composition in which the light energy absorptioncoefficient is intensity or fluence dependent, where intensity is energyper unit area per unit time and fluence is energy density or energy perunit area.

The term “color forming compound” as used herein refers to compoundsthat upon heating produce a detectable color change. A color change maybe detectable either with the naked eye or by using optical devices,such as, for example, a camera or microscope. Typically the color changeis from a colorless or lightly colored state to a more intensely coloredstate. The detectable color change may be the result of the applicationof heat alone or may be a combination of heat and interaction with otherreagents.

The term “leuco dye” as used herein refers to compounds which changefrom essentially colorless to colored when heated, with or without thepresence of other reagents.

The term “light source” as used herein refers to a source of radiation,for example, in the wavelength range of about 300-1500 nanometers. Insome embodiments, the light source may be a laser. Lasers are well-knownas sources of amplified, coherent electromagnetic radiation. In someembodiments, the laser may be combined with focusing optics to focus thelaser output to a specific location along the z axis. As used herein,the term “z axis” refers to the depth or thickness of the multi-layerconstruction material, where the x-y plane defines the top or bottomsurface of the material. In some embodiments the wavelength of the lightsource radiation is in the range 350-850 nanometers.

The term “fixing” as used herein refers to a post-imaging process stepinvolving either exposure to radiation or heat to inactivate the colorforming compound and prevent further imaging.

The term “image” as used herein refers to any pattern of opticalcontrast produced on the inside of a substrate that has a similarappearance to a person or object. The image can have a two-dimensionalor three-dimensional appearance. The image can be observed directly withthe naked eye or by an instrument such as an optical device like acamera or microscope. Text and indicia, such as for example a name,birth date, signature, employee number, social security number and thelike as well as decorative patterns and designs are, for purposes ofthis disclosure, also considered to be images. A “full color image” isone which contains the colors cyan, magenta, yellow and black. A“durable image” is one which upon imaging retains its consistency forthe desired lifetime of use of the article bearing the image. In someuses the desired lifetime may be relatively long such as one year, 5years, or even 10 years or more. A “fixed image” is one to which afixing step has been carried out.

The term “voxel” as used herein refers to a volume element in threedimensional space. The term is analogous to a pixel in two dimensionalspace.

The term “alkyl” refers to a monovalent group that is a radical of analkane, which is a saturated hydrocarbon. The alkyl can be linear,branched, cyclic, or combinations thereof and typically has 1 to 20carbon atoms. In some embodiments, the alkyl group contains 1 to 18, 1to 12, 1 to 10, 1 to 8, 1 to 6, or 1 to 4 carbon atoms. Examples ofalkyl groups include, but are not limited to, methyl, ethyl, n-propyl,isopropyl, n-butyl, isobutyl, tert-butyl, n-pentyl, n-hexyl, cyclohexyl,n-heptyl, n-octyl, and ethylhexyl.

The term “aryl” refers to a monovalent group that is aromatic andcarbocyclic. The aryl can have one to five rings that are connected toor fused to the aromatic ring. The other ring structures can bearomatic, non-aromatic, or combinations thereof. Examples of aryl groupsinclude, but are not limited to, phenyl, biphenyl, terphenyl, anthryl,naphthyl, acenaphthyl, anthraquinonyl, phenanthryl, anthracenyl,pyrenyl, perylenyl, and fluorenyl.

The term “heteroalkyl” refers to an alkyl group which containsheteroatoms such as N, O, S or halogens (F, Cl, Br, I).

The disclosed methods for generating an image utilize multi-layerconstructions. The multi-layer constructions may be prepared using avariety of techniques used to prepare multi-layer constructions. Forexample, the multi-layer construction may be prepared directly bysequential coating or laminating steps. Sequential coating steps mayinclude using coating techniques to form the individual layers withinthe multi-layer construction. An individual layer may be formed bycoating a film-forming composition and permitting the layer to form by,for example, drying if a solvent is used with the film-formingcomposition. Examples of useful coating techniques include, for example,die coating, knife coating, roll coating, gravure coating, rod coating,curtain coating, air knife coating and printing techniques such asscreen printing or inkjet printing. The film forming composition may becoated onto a carrier film which may or may not become incorporated intothe final construction or the layers may be sequentially coated to formthe multi-layer construction.

Alternatively, the individual layers can be prepared separately andlaminated together. For example, a series of individual layers may beprepared separately by coating techniques as described above and theseparately formed layers may be laminated together to form themulti-layer construction. Lamination may involve the use of pressure,heat, adhesives or a combination thereof. Since at least some of thelayers are thermally activatable, heat should be used cautiously toavoid premature activation of these layers. The layers may be adheredtogether across the entire surface of the layer or at selected points,such as at the edges.

In some embodiments it may be desirable to use a combination oftechniques. For example, if thermally non-activatable layers (i.e.layers or films that do not comprise thermally activatable compositions)are present between, or on top of or below, the thermally activatablelayers of the construction, it may be desirable to coat a film-formingcomposition containing the components of the thermally activatablecomposition dissolved or suspended in a solvent onto a thermallynon-activatable layer. Upon drying, the resulting layer which is now athermally activatable layer, can be laminated to other layers, coveredwith a thermally non-activatable film or coated with anotherfilm-forming composition to continue to build up the multi-layerconstruction.

The thermally activatable layers typically comprise a polymeric binderin addition to the non-linear light to heat converter composition andthe color changing compound. Any suitable polymeric material may be usedas the polymeric binder as long is doesn't interfere with the imageformation, observation or stability. In some embodiments, a thermallyactivatable layer may comprise from 50 wt % to 99.8 wt % polymericbinder.

A wide variety of non-reactive polymeric binders may be used. Thepolymeric binders are useful, for example, to control viscosity and toprovide film-forming properties. Such polymeric binders typically arechosen to be compatible with the thermally activatable materials. Forexample, polymeric binders that are soluble in the same solvent orsolvents that are used for the thermally activatable materials and thatare free of functional groups that can adversely affect the color changeof the thermally activatable species can be utilized. Polymeric bindersmay be of any suitable molecular weight to achieve the desired solutionrheology and film-forming properties. In some embodiments, the numberaverage molecular weight, Mn, may be between 5,000 and 1,000,000Daltons, or 10,000 to 500,000 Daltons or even 15,000 to 250,000 Daltons.Suitable polymeric binders include, for example, polystyrenes,poly(methyl methacrylates), poly(styrene)-co-(acrylonitriles), celluloseacetate butyrates, poly(bisphenol A epichlorohydrin)glycidyl end-cappedcopolymers and the like.

Generally the film-forming composition that comprises the non-linearlight to heat converter composition, the color forming compound and theoptional polymeric binder also contains a solvent. After coating thesolvents are allowed to dry to give the thermally activatable layer.Suitable solvents include, for example: esters such as ethyl acetate orpropylene glycol methyl ether acetate; ethers such as tetrahydrofuranand diethyl ether; ketones such as acetone, cyclopentanone and methylethyl ketone; alkanes such as hexane, heptane, and petroleum ether;arenes such as benzene and toluene; alcohols such as methanol, ethanoland isopropanol; and mixtures thereof.

Typically the multi-layer construction comprises at least one thermallyactivatable layer. In some embodiments the multi-layer constructioncomprises 2 or more thermally activatable layers, for example twothermally activatable layers, three thermally activatable layers or fourthermally activatable layers. The thermally activatable layers may bethe same or different, typically they are different to produce differentcolors upon activation. To achieve full color images, the multi-layerconstruction may comprise three thermally activatable layers whichproduce the colors cyan, yellow and magenta upon activation or fourthermally activatable layers, which produce the colors cyan, yellow,magenta and black upon activation.

The thermally activatable layers may have a variety of thicknesses.Generally the layer thickness is selected to be thick enough to bereadily selectively activated and to generate sufficient color to createa clear image. Typically the thermally activatable layers have athickness of from about 2-50 micrometers, or about 5-25 micrometers.

Generally the thermally activatable layer comprises a single layer ofmaterial, but the thermally activatable layer may comprise sublayers. Inthis context, sublayers are continuous or discontinuous layers thattogether form the thermally activatable layer. For example, it may bedesirable in some embodiments to prepare a thermally activatable layerwhich comprises separate sublayers of color forming compound andnonlinear light to heat converter composition. These thermallyactivatable sublayers may be separated by other sublayers comprising athermally sensitive material, such as a wax. In this way, the thermallyactivatable sublayers are kept separated until activation by a lightsource. Such a sublayer configuration could be prepared by separatelycoating layers of non-linear light to heat converter composition andcolor forming compound and optional wax, instead of mixing thenon-linear light to heat convertor composition and color formingcompound and coating the mixture. Such a sublayer configuration isdescribed, for example, in U.S. Pat. No. 6,801,233 (Bhatt et al.) andU.S. Pat. No. 7,166,558 (Bhatt et al.), and the sublayers of thermallysensitive material can help provide an additional physical barrier toaccidental activation of the color forming compound in the thermallyactivatable sublayers by heat other than the heat generated by thedesired activation by a light source. Upon activation, the wax melts andthe sublayers interact and generate the desired color.

The multi-layer construction may include both thermally activatable aswell as thermally non-activatable layers, or all of the layers may bethermally activatable. Typically, the exterior layer or layers of theconstruction are thermally non-activatable with at least one internallayer being thermally activatable. If a three dimensional image isdesired, more than one thermally activatable layer should be present inthe multi-layer construction.

Additionally there may be thermally non-activatable layers betweenthermally activatable layers. The presence of thermally non-activatablelayers between thermally activatable layers may help prevent thethermally activatable layers from interacting during preparation orduring activation.

The thermally non-activatable layers, if used, may be prepared from anyuseful materials. Typically the layers comprise polymeric materials. Insome embodiments the polymeric materials are thermoplastics, butelastomeric materials may also be used in certain embodiments.Generally, thermally non-activatable layers positioned above thethermally activatable layers in a multi-layer construction aretransparent to visible light thus permitting the image formed in thethermally activatable layers to be visible. Thermally non-activatablelayers that are below the image or not in the light path of the imageneed not be transparent to visible light.

Thermally non-activatable layers may be used to provide rigidity to themulti-layer construction and/or protection for the image formed by thethermally activatable layers. For example, the non-activatable layersmay be used to provide chemical resistance, scratch resistance, shockresistance and tamper resistance for the image. The thermallynon-activatable layers may exist as a single layer or multiple layers.If multiple layers are used, the multiple layers may comprise differentcompositions.

Examples of useful materials for thermally non-activatable layersinclude, for example, polyesters, polyurethanes, polyolefins, andpolycarbonates.

The thickness of thermally non-activatable layers may be any suitablethickness. In some embodiments, the thermally non-activatable layers mayvary from 5 micrometers to 50 micrometers in thickness. In someembodiments, for example it may be desirable to have relatively thick(that is to say greater than 50 micrometers) thermally non-activatablelayers to provide rigidity and resiliency to the multi-layerconstruction. In other embodiments it may be desirable to haverelatively thin thermally non-activatable layers. In still otherembodiments it may be desirable to have some thermally non-activatablelayers be relatively thick (such as, for example, the exterior layers ofthe multi-layer construction) and have other layers be relatively thin(such as, for example, the layers between the thermally activatablelayers).

The thermally activatable layers include a non-linear light to heatconverter composition. The light absorption through a medium followsBeer's Law as shown in equations 1 and 2 below:

$\begin{matrix}{{{I(z)} = {I_{0}{\mathbb{e}}^{{- \alpha}\; z}}}{or}} & (1) \\{\frac{\mathbb{d}I}{\mathbb{d}z} = {{- \alpha}\; I}} & (2)\end{matrix}$

where I(z) is the intensity of the light at the medium penetration depthz, I₀ is the light intensity before it is attenuated by the medium, α isthe absorption coefficient and I is the light intensity. For a linearlight absorber, the absorption coefficient is a constant, independent ofthe light intensity or fluence. Nonlinear absorption, on the other hand,means that the absorption coefficient depends on the light intensity orfluence. This can lead to both increased transmittance with increasinglight intensity or decreased transmittance with increasing lightintensity.

A wide variety of non-linear light to heat converter compositions areuseful. The nonlinear light absorbers may be two-photon absorption (2Ph)species or reverse saturable absorption (RSA) species.

The two-photon (2Ph) process is a nonlinear light absorption process inwhich the photon energy is approximately equal to half the energyrequired for linear excitation of the material. Excitation of theabsorbing material therefore requires the simultaneous absorption of twoof the lower energy photons. The absorption coefficient for this processis therefore light intensity dependent as shown in equation 3 below:α=α₀ +βI  (3)

where α₀ is the residual linear absorption coefficient and β is thetwo-photon absorption coefficient.

Examples of useful two-photon absorbers include those exhibiting largemultiphoton absorption cross-sections, such as Rhodamine B (that is,N-[9-(2-carboxyphenyl)-6-(diethylamino)-3H-xanthen-3-ylidene]-N-ethylethanaminiumchloride and the hexafluoroantimonate salt of Rhodamine B) and the fourclasses of photosensitizers described, for example, in PCT PublicationNos. WO 98/21521 and WO 99/53242. The four classes can be described asfollows: (a) molecules in which two donors are connected to a conjugatedπ-electron bridge; (b) molecules in which two donors are connected to aconjugated π-electron bridge which is substituted with one or moreelectron accepting groups; (c) molecules in which two acceptors areconnected to a conjugated π-electron bridge; and (d) molecules in whichtwo acceptors are connected to a conjugated π-electron bridge which issubstituted with one or more electron donating groups (where “bridge”means a molecular fragment that connects two or more chemical groups,“donor” means an atom or group of atoms with a low ionization potentialthat can be bonded to a conjugated π-electron bridge, and “acceptor”means an atom or group of atoms with a high electron affinity that canbe bonded to a conjugated π-electron bridge).

The four above-described classes of two-photon absorbers can be preparedby reacting aldehydes with ylides under standard Wittig conditions or byusing the McMurray reaction, as detailed in PCT Publication No. WO98/21521.

Other useful two-photon absorbers are described in U.S. Pat. Nos.6,100,405, 5,859,251, and 5,770,737. These compounds are described ashaving large multiphoton absorption cross-sections.

The RSA process is also sometimes referred to as excited stateabsorption, and is characterized by the absorption cross section for theexcited state involved in the absorption process being much larger thanthe cross section for excitation from the ground state to the excitedstate. The total light absorption involves both ground state absorption(the linear term) and excited state absorption. The equation describingthe light intensity (I) as a function of depth of the material (z) istherefore given by equation 4:

$\begin{matrix}{\frac{\mathbb{d}I}{\mathbb{d}z} = {{- \left( {\alpha_{0} + {\sigma\; N}} \right)}I}} & (4)\end{matrix}$

where σ is the absorption cross-section for transitions from the excitedstate. The population density of the excited state N, is produced byground state absorption (linear absorption) with absorption coefficientα₀ and is given by equation 5 below:dN/dt=α ₀ I/ηω  (5)

where ℏω is the incident photon energy.

Integration of equation (5) with respect to time, substitution intoequation (4), and integration with respect to time again results inequation 6 below:

$\begin{matrix}{\frac{\mathbb{d}F}{\mathbb{d}z} = {{- \left\lbrack {\alpha_{0} + {\frac{\alpha_{0}\sigma}{2{\eta\omega}}F}} \right\rbrack}F}} & (6)\end{matrix}$

These equations show that the dependence of the energy density, orfluence (F), of the light in the material depends on the square of thefluence. This equation is analogous to equation 3 above, with the term βreplaced by α₀σ/2ℏω, and indicates that excited state absorption andtwo-photon absorption generally give nearly identical results forabsorption of light by the material as a function of light intensity orfluence.

Examples of reverse saturable absorption materials that function asnon-linear light to heat converter compositions include, for example,metallophthalocyanines, naphthalocyanines, cyanines, fullerenes, metalnanoparticles, metal oxide nanoparticles, metal cluster compounds,porphyrins, indanthrone derivatives and oligomers or combinationsthereof. Examples of metallophthalocyanines include, for example, copperphthalocyanine (CuPC), and phthalocyanines containing metal ormetalloids from group IIIA (Al, Ga, In) and IVA (Si, Ge, Sn, Pb).Examples of naphthalocyanines include, for example, the phthalocyaninederivatives of silicon (SiNC), tin (SnNC), and lead (PbNC). Examples ofcyanines include, for example,1,3,3,1′,3′,3′-hexamethylindotricarbocyanine iodide (HITCI). Examples offullerenes include, for example, C60 and C70 fullerenes. Examples ofmetal nanoparticles include, for example, gold, silver, platinum,aluminum, and zinc nanoparticles, Examples of metal oxide nanoparticlesinclude, for example, titanium dioxide, antimony tin oxide, andzirconium dioxide nanoparticles. Examples of metal clusters include, forexample, iron tricobalt metal clusters such as HFeCO₃(CO)₁₂ andNEt₄FeCO₃(CO)₁₂. Examples of porphyrins include, for example,tetraphenylporphyrin (H2TPP), zinc tetraphenylporphyrin (ZnTPP), andcobalt tetraphenylporphyrin (CoTPP). Examples of indanthrone derivativesinclude, for example, unsubstituted indanthrone, oxidized indanthrone,chloroindanthrone, and an indanthrone oligomer.

In some embodiments the non-linear light to heat converter compositioncomprises copper phthalocyanine, tin phthalocyanine, or a combinationthereof.

The non-linear light to heat converter composition generally may bepresent in fairly small quantities. Typically the non-linear lightabsorber is present in the amount of about 0.05-5 weight % or even 0.1-3weight %.

The thermally activatable layers also include a color forming compound.The color forming compounds typically are cyan-forming (i.e. forms acyan color upon activation), magenta-forming (i.e. forms a magenta colorupon activation), yellow-forming (i.e. forms a yellow color uponactivation), or black-forming (i.e. forms a black color uponactivation). Typically the color forming compound is a leuco dye. Leucodyes are compounds which change from essentially colorless to colored bythe input of heat, with or without the presence of other reagents.

A number of classes of leuco dye materials are useful as the colorforming compounds of this disclosure. Among the useful materials are,for example: azines such as oxazines, diazines and thiazines;triarylmethanes such as fluoresceins, rhodamines and rhodols; ketazines;barbituric acid leuco dyes and thiobarbituric acid leuco dyes.

Examples of azine leuco dyes that are suitable for use as color formingcompounds include those which can be described by Formula I below:

where R¹ is —C(O)R², where C(O) designates a carbonyl group, each X¹ canindependently be OR², NR² ₂ or SR² and Y¹ can be O, NR² or S, where eachR² is independently a hydrogen, an alkyl, an aryl, or a heteroalkyl. InFormula I, when Y¹ is an oxygen the compound is an oxazine, when Y¹ isequal to NR², the compound is a diazine and when Y¹ is equal to sulfurthe compound is a thiazine. In some embodiments the color formingcompound of Formula I has X¹ equal to NR² ₂, where R² is an alkyl, Y¹ isequal to oxygen, and R¹ is equal to —C(O)Ar, where Ar is an aryl group.In some embodiments the color forming compound of Formula I has X¹ equalto NR² ₂, where R² is an ethyl group, Y¹ is equal to oxygen, and R¹ isequal to —C(O)Ph, where Ph is a phenyl group.

Examples of triarylmethane leuco dyes include those which can bedescribed by Formula II below:

where X² can be O, NR² or S, where R² is a hydrogen, an alkyl, an aryl,or a heteroalkyl, X³ can be carbonyl (C═O), thiocarbonyl (C═S), or SO₂,and each Y² can independently be OR², NR² ₂ or SR² where R² is asdefined above. In Formula II, when each Y² is OR², the compound is afluorescein, when each Y² is NR² ₂, the compound is a rhodamine and whenone Y² is NR² ₂ and the other Y² is OR² the compound is a rhodol. Insome embodiments, X³ is a carbonyl, X² is O or S, and Y² is OR², NR² ₂or SR² where R² is as defined above. In some embodiments, X³ is acarbonyl, X² is O, and Y² is OR², NR² ₂ or SR² where R² is as definedabove.

Examples of ketazines include those which can be described by FormulaIII below:

where each R² is independently a hydrogen, an alkyl, an aryl, or aheteroalkyl. In some embodiments, each R² is independently a hydrogen oran alkyl group.

Examples of barbituric acid leuco dyes include those which can bedescribed by Formula IV below:

where each R² is independently a hydrogen, an alkyl, an aryl, or aheteroalkyl. In some embodiments, each R² is independently a hydrogen oran alkyl group.

Examples of thiobarbituric acid leuco dyes include those which can bedescribed by Formula V below:

where each R² is independently a hydrogen, an alkyl, an aryl, or aheteroalkyl. In some embodiments, each R² is independently a hydrogen oran alkyl group.

In addition to the above classes of leuco dyes, materials described byFormula VI below may also be useful leuco dyes:

where each R² is independently a hydrogen, an alkyl, an aryl, or aheteroalkyl, each X⁴ is independently hydrogen, OR², NR² ₂ or SR², whereR² is as defined above, each X⁵ is independently hydrogen, NO₂ or CN. Insome embodiments, each R² is independently a hydrogen or an alkyl group,each X⁴ is OR³, where R³ is hydrogen or an alkyl group, and X⁵ is NO₂.In some embodiments, the X⁴ of one ring is OR³ where R³ is an ethylgroup, and the X⁴ of the other ring is hydrogen, the X⁵ of one ring ishydrogen and the X⁵ of the other ring is NO₂, and each R² is tert-butyl.

where X¹ is OR², NR² ₂ or SR², X³ can be carbonyl (C═O), thiocarbonyl(C═S), or SO₂, and each R² is independently a hydrogen, an alkyl, anaryl, or a heteroalkyl as previously defined above, and X⁶ is OR², NR² ₂or SR², X⁷ is hydrogen, OR², or SR², and X⁸ is R², OR², NR² ₂ or SR². Insome embodiments X³ is carbonyl (C═O), or SO₂, X⁶ is OR², NR² ₂ or SR²,X⁷ is hydrogen, OR², or SR², and X⁸ is R², OR², NR² ₂ or SR².

The amount of color forming compound included in the thermallyactivatable composition varies depending upon the specific color formingcompound chosen as well as other factors, such as desired colorintensity, cost, etc. Typically the color forming compound is present inthe amount of about 0.1-10 weight % or even 0.5-5 weight %.

In addition to the non-linear light to heat converter composition, thecolor forming compound and the optional polymeric binder, the thermallyactivatable layer may also contain other additives. Among the usefuladditives which may be incorporated into the thermally activatablelayer, are species which aid the image formation. For example, thermalacid generators may be included. The thermal acid generators release anacid upon thermal excitation. The acid can initiate or catalyze thethermal transformation of the color forming compound permitting morerapid and/or more complete image formation. Suitable thermal acidgenerators include both materials that generate Bronsted acids (protons)and Lewis acids (electron pair acceptors).

A variety of materials which liberate acid upon heating may be used as athermal acid generator. For example, Sabongi, G. J., ChemicalTriggering-Reactions of Potential Utility in Industrial Processes,Plenum Press, New York, N.Y. (1987), pages 68-72 describes thermallytriggered release of carboxylic acids from esters and oxime derivatives,especially benzaldoximes and oxalic acid esters.

Examples of useful thermal acid generators include those disclosed inU.S. Pat. No. 4,670,373 (Kitaguchi et al.), which describes the thermaldecomposition of N-carboxy aldoximes to give benzoic acid derivatives asshown below.

Another class of useful thermal acid generators includes those disclosedin U.S. Pat. No. 4,011,352 (Janssens et al.) which describes the thermaldecomposition of the half esters of anhydrides to give strongerdicarboxylic acids as is shown below.

Examples of Lewis acid thermal acid generators include those disclosedin U.S. Pat. No. 2,995,466 (Sorensen) which describes the thermalgeneration of Lewis acids from diazonium salt acid-progenitorscontaining counterions such as tetrafluoroborate, hexafluorophosphate,and the like. An example of the acid generation mechanism is shownbelow.

Another example of suitable thermal acid generators includes thosedisclosed in U.S. Pat. No. 5,554,664 (Lamanna, et al.) which describesthe thermal generation of acids from diazonium salts withnon-coordinating anionic counterions. Examples of anionicnon-coordinating anions include for example highly fluorinatedalkylsulfonyl methide, a fluorinated arylsulfonyl methide, a highlyfluorinated alkyl sulfonyl imide, a fluorinated arylsulfonyl imide, amixed aryl- and alkylsulfonyl imides and methides, or combinationsthereof.

Another type of suitable thermal acid generator is that described inU.S. Pat. No. 5,278,031 (Boggs et al.), which describes certain squaricacid derivatives as effective heat-sensitive acid generating materialsfor use in thermal imaging. An example of this thermal transformation isshown below. Squaric acid has reported pKa values of 0.59 and 3.48(Schwartz and Howard, J. Phys. Chem. 74 4374 1970).

An additional example of a suitable thermal acid generator includes thedisclosure of U.S. Pat. No. 5,395,736 (Grasshoff et al.), whichdescribes the exposure of superacid precursors to actinic radiation togenerate superacids, which can catalyze the thermal decomposition ofsecondary acid precursors such as squaric acid derivatives or oxalicacid derivatives in admixture with the superacid precursor, thusincreasing the quantity of strong acid present in an imaging medium.Known superacid precursors include diazonium, phosphonium, sulfonium andiodonium compounds. Similarly, U.S. Pat. No. 5,914,213 (Grasshoff etal.) describes a similar process using tosylate and phosphatederivatives as secondary acid precursors.

Another suitable class of thermal acid generators is that disclosed inU.S. Pat. No. 6,627,384 (Kim et al.), which describes cyclic alcoholswith adjacent sulfonate leaving groups. The sulfonate leaving groupsform acids upon the application of heat as is demonstrated in themechanism shown below.

Typically, if used, the thermal acid generator is present in thethermally activatable layer in amounts of about 0.1-10 weight %.

Additionally, the thermally activatable layer may contain fixingadditives. Fixing additives are added to fix the image once it isformed. By fixing the image, the image is prevented from undergoingfurther imaging to alter, obscure, obliterate or otherwise change theimage. Examples of fixing additives include, for example,1-phenyl-3-pyrazolidone, hydroquinones, naphthoquinones, a hydroquinoneethers, naphthoquinone ethers, hydronaphthoquinone ethers, or mixturesthereof.

Typically, if used, the fixing compound is present in the thermallyactivatable layer in amounts in the range of about 0.05-10 weight %.

FIG. 1 shows a schematic cross section of an embodiment of a multi-layerimage-forming construction capable of forming full color images.Multi-layer construction 100 comprises thermally activatable layers 20,30, 40, and 50. In this embodiment, layer 20 is a cyan thermallyactivatable layer, layer 30 is a magenta thermally activatable layer,layer 40 is a yellow thermally activatable layer, and 50 is a blackthermally activatable layer. Layer 10 is a thermally nonactivatablelayer which covers the image formed within the layers 20, 30, 40 and 50.Optional layer 60 is a thermally nonactivatable layer which is below theimage and serves as a support layer. Optional layers 15, 25, 35, 45, and55 are thermally nonactivatable layers which separate the other layers.

Also disclosed are articles which are multi-layer constructions whichcontain a color image. The images typically are formed by selectivelyactivating one or more thermally activatable layers within themulti-layer construction with a light source. The thermally activatablelayers have been described above. Generally the activating light sourceis a collimated light source such as a laser Laser light sources areparticularly useful because they are able to provide localizedactivation, that is, selective activation of a voxel on a single layerwithin multiple layers. Particularly useful are lasers that are combinedwith focusing optics to focus the laser output to a specific locationalong the z axis.

It is desirable that a single laser be able to activate all of thethermally activatable layers. It is also desirable that all thermallyactivatable layers be activated in a single pass with that laser.

Localized activation, such as activation of a single voxel within alayer, aids in the formation of well-resolved images when the layerswithin a multi-layer construction form different colors upon activation.

Unlike other multi-layer systems in which each layer is responsive to adifferent laser wavelength, the multi-layer construction of thisdisclosure permits color image formation with a single laser. Theselective activation is achieved through the use of focusing optics tofocus the laser output to a specific location along the z axis. In someembodiments, the laser pulse duration is from 100 picoseconds to 1microsecond.

Many lasers emit beams with a Gaussian profile, in which case the laseris said to be operating on the fundamental transverse mode, or “TEM₀₀mode” of the laser's optical resonator. The Gaussian beam is a beam ofelectromagnetic radiation whose transverse electric field and intensity(irradiance) distributions are described by Gaussian functions. For aGaussian beam, the spot size w(z) is defined as the radius of the circlearound the z axis when the intensity at the circle is 1/e² to that ofthe center of the circle. When an incident laser beam is focused by alens, the propagated laser beam spot size w(z) will be at a minimumvalue w₀ at one place along the beam propagation axis z, known as thebeam waist.

For a beam of wavelength λ at a distance z along the beam from the beamwaist, the variation of the spot size is given by equation 7:

$\begin{matrix}{{w(z)} = {w_{0}\sqrt{1 + \left( \frac{z}{z_{0}} \right)^{2}}}} & (7)\end{matrix}$

where the origin of the z-axis is defined, without loss of generality,to coincide with the beam waist, and where z₀ is called the Rayleighrange and is defined by equation 8:

$\begin{matrix}{z_{0} = {\frac{\pi\; w_{0}^{2}}{\lambda}.}} & (8)\end{matrix}$

At a distance from the beam waist equal to the Rayleigh range z₀, thewidth, w, of the beam is defined by equation 9:w(±z ₀)=w ₀√{square root over (2)}  (9).

The distance between these two points (+z₀ and −z₀) is called theconfocal parameter, b, or the depth of focus of the beam, and is 2 timesthe Rayleigh range z₀.

Another way to describe the laser focus is by the numerical aperture(usually abbreviated NA), which is the refractive index of the lensmaterial, n, times the sine of the half angle of the cone of the focus Θas shown in equation 10 below:

$\begin{matrix}{{NA} = {{n*\sin\frac{\Theta}{2}} \approx \frac{D}{2\; f}}} & (10)\end{matrix}$

where f is the focal length of the focusing lens and D is the diameterof the lens or the diameter of the laser beam coming into the lens.

The spot size of the focused beam is determined by the numericalaperture defined in equation 10. If the beam incident on the lens has aconstant transverse profile, i.e., a flat top beam, the focused spot hasan intensity profile described by the Airy disc where the diameter(2w_(o)) of the first dark ring is given by equation 11 below:

$\begin{matrix}{{2\; w_{0}} = {1.22\frac{\lambda}{NA}}} & (11)\end{matrix}$

where λ is the laser wavelength. The focused spot size is inverselyrelated to the NA. A comparison of equations 9 and 11, demonstrates thatthe depth focus of the beam is directly related to the laser wavelengthand inversely related to the square of NA.

In this disclosure the laser beam is focused along the z axis, with thebeam waist located at the approximate location of a specific thermallyactivatable layer. Whereas linear absorption processes vary linearlywith the light intensity or fluence, the nonlinear absorption processescharacteristic of the materials used in this disclosure vary as theintensity or fluence raised to some higher power (such as a quadraticdependence). With a linear absorption process, it is generally notpossible to independently activate individual thermally activatablelayers, even by focusing the laser beam waist in the middle of thedesired thermally activatable layer with a high numeric aperture lens(NA≧0.3). This is due to the resulting depth of focus of the beam beingcomparable to or greater than typical layer thicknesses. However, whenthe laser beam waist is located at the location of a thermallyactivatable layer using the nonlinear intensity or fluence dependence ofthe absorption processes characteristic of the method of thisdisclosure, that layer can be activated without significant activationof other thermally activatable layers. This is the result of theeffective distance along the z-axis of the nonlinear absorption processbeing less than the depth of focus of the laser beam. The laser beamintensity or fluence rapidly becomes too low to activate the nonlinearlight-to-heat conversion process in adjacent thermally activatablelayers.

Among the articles of this disclosure are, for example, a wide varietyof identification documents (ID documents). The term “ID documents” isbroadly defined and is intended to include, but not be limited to, forexample, passports, driver's licenses, national ID cards, socialsecurity cards, voter registration and/or identification cards, birthcertificates, police ID cards, border crossing cards, security clearancebadges, security cards, visas, immigration documentation and cards, gunpermits, membership cards, phone cards, stored value cards, employeebadges, debit cards, credit cards, and gift certificates and cards. Thearticles of this disclosure may be the ID document or may be part of theID document. Other articles containing color images included in thisdisclosure include items of value, such as, for example, currency, banknotes, checks, and stock certificates, where authenticity of the item isimportant to protect against counterfeiting or fraud, as well asarticles which can be used to produce informative, decorative, orrecognizable marks or indicia on product tags, product packaging,labels, charts, maps and the like.

The articles of this disclosure are particularly useful in the formationof ID documents because they permit the formation of a color image thatis contained within a multi-layer construction. The multi-layerconstruction provides for protection of the image from damage,environmental degradation and tampering.

EXAMPLES

These examples are merely for illustrative purposes only and are notmeant to be limiting on the scope of the appended claims. All parts,percentages, ratios, etc. in the examples and the rest of thespecification are by weight, unless noted otherwise. Solvents and otherreagents used were obtained from Sigma-Aldrich Chemical Company;Milwaukee, Wis. unless otherwise noted.

Table of Abbreviations

Abbreviation or Trade Designation Description Dye-1 A cyan-colored leucodye, 10-benzoyl-N,N,N′,N′- tetraethyl-10H-phenoxazine-3,7-diamine,prepared as described in German Pat. No. DE 2,237,833. CuPC Copperphthalocyanine Binder-1 PKHH, poly (bisphenol A epichlorohydrin)glycidyl end-capped copolymer with Mn 1,075. THF Tetrahydrofuran Dye-2yellow leuco dye, 2,6-di-tert-butyl-4-[5-(4-ethoxy-3-nitrophenyl)-4-phenyl-1H-imidazol-2-yl]-phenol, prepared as describedbelow in Synthesis Example S1. Dye-3 magenta leuco dye prepared asdescribed in U.S. Pat. No. 4,563,415.

Synthesis Example S1 Preparation of Yellow Leuco Dye Dye-2 Step 1:Preparation of 1-(4-ethoxyphenyl)-2-phenylethane-1,2-dione

The synthesis of 1-(4-ethoxyphenyl)-2-phenylethane-1,2-dione was carriedout as described in Nelson, J. Leonard, Richard T. Rapala, Hershel L.Herzog and Elkan R. Blout; J. Am. Chem. Soc.; 71; 1949; 2997-2998. Thesynthesis is summarized by the equation:

Step 2: Preparation of1-(4-ethoxy-3-nitrophenyl)-2-phenylethane-1,2-dione

To 0.5 gram of 1-(4-ethoxyphenyl)-2-phenylethane-1,2-dione prepared inStep 1, was add 10 milliliters of concentrated nitric acid and theresulting mixture was heated to 110° C. for 2 hours. The reactionmixture was cooled and poured into 20 milliliters of water. Theprecipitate was recrystallized from ethanol to give1-(4-ethoxy-3-nitrophenyl)-2-phenylethane-1,2-dione. The synthesis issummarized by the equation:

Step 3: Preparation of2,6-di-tert-butyl-4-[5-(4-ethoxy-3-nitrophenyl)-4-phenyl-1H-imidazol-2-yl]-phenol

A mixture of 10.8 grams if 3,5-di-tert-butyl-4-hydroxybenzaldehyde(commercially available from Alfa Aesar), 11.7 grams of1-(4-ethoxy-3-nitrophenyl)-2-phenylethane-1,2-dione (prepared in Step 2above), and 27.5 grams of ammonium acetate in 200 milliliters of aceticacid was heated to reflux for 3 hours. The mixture was cooled and pouredinto water and the precipitate was collected by filtration and then airdried overnight to give2,6-di-tert-butyl-4-[5-(4-ethoxy-3-nitrophenyl)-4-phenyl-1H-imidazol-2-yl]-phenol.The synthesis is summarized by the equation:

Example 1

The dinitrate salt of Dye 1 was prepared by adding 0.43 grams (1.001millimoles, 1 eq) of Dye 1 to 60 milliliters of diethyl ether cooledwith ice and adding dropwise a solution of 0.1252 milliliters of a 15.99M solution (2.002 millimoles, 2 eq) of nitric acid in 16 M 10milliliters of diethyl ether. A white precipitate formed immediately.The mixture was stirred in the ice bath for a few minutes and the whitesolid was collected, washed with ether, and dried under a vacuum to givethe dihydronitrate of Dye 1. A solution containing 1% by weight of thedihydronitrate salt of Dye-1, 1% by weight CuPC, and 20% by weight ofBinder-1 in THF was prepared. The solution was solvent coated on a glassplate with a #20 Meyer rod and the solvent was allowed to dry. A diodepumped Nd:YAG laser (commercially available from IB Laser Berlin,Germany) at the second harmonic wavelength (532 nanometers), with pulseduration of 10 nanoseconds, and a pulse repetition rate of 1,000 Hz wasused to mark the dye-coated sample. The output of the laser was coupledto an optical fiber, collimated by a lens with a focal length of 150millimeters and focused on the sample by a lens with a focal length of30 millimeters. This assembly was held by a robot arm with the laserfocal spot a distance of 6 millimeters away from the sample. The laserwas moved at 1 millimeter per second and the single pulse energy fromthe laser was either 1.0 milliJoules or 0.5 milliJoules at the sample.The cyan lines were successfully engraved.

Comparative Example C1

A solution containing 1% by weight of Dye-1, and 20% by weight ofBinder-1 in THF was prepared. The solution was solvent coated on a glassplate with a #20 Meyer rod and the solvent was allowed to dry. A diodepumped Nd:YAG laser (commercially available from IB Laser Berlin,Germany) at the fundamental wavelength (1.064 micrometers), with pulseduration of 10 nanoseconds, and a pulse repetition rate of 400 Hz wasused to mark the dye-coated sample. The output of the laser was coupledto an optical fiber, collimated by a lens with a focal length of 150millimeters and focused on the sample by a lens with a focal length of30 millimeters. This assembly was held by a robot arm with the laserfocal spot a distance of 6 millimeters away from the sample. The laserwas moved at 1 millimeter per second and the single pulse energy fromthe laser was 1.5 milliJoules at the sample. No color change wasobserved.

Example 2

A solution of 20 weight % solids in THF was prepared containing 400milligrams of Binder-1, 40 milligrams phthalic acid, 40 milligrams ofDye-1, 40 milligrams of magnesium nitrate, and 0.5 milligrams of CuPC.The solution was solvent coated on a glass plate with a #20 Meyer rodand the solvent was allowed to dry. A diode pumped Nd:YAG laser(commercially available from IB Laser Berlin, Germany) at the secondharmonic wavelength (532 nanometers), with pulse duration of 10nanoseconds, and a pulse repetition rate of 1,000 Hz was used to markthe dye-coated sample. The output of the laser was coupled to an opticalfiber, collimated by a lens with a focal length of 150 millimeters andfocused on the sample by a lens with a focal length of 30 millimeters.This assembly was held by a robot arm with the laser focal spot adistance of 6 millimeters away from the sample. The laser was moved at 1millimeter per second and the single pulse energy from the laser waseither 1.0 milliJoules or 0.5 milliJoules at the sample. Cyan lines weresuccessfully engraved.

Example 3

A solution of 20 weight % solids in THF was prepared containing 400milligrams of Binder-1, 40 milligrams phthalic acid, 40 milligrams ofDye-3, 40 milligrams of magnesium nitrate, and 0.5 milligrams of CuPC.The solution was solvent coated on a glass plate with a #20 Meyer rodand the solvent was allowed to dry. A diode pumped Nd:YAG laser(commercially available from IB Laser Berlin, Germany) at the secondharmonic wavelength (532 nanometers), with pulse duration of 10nanoseconds, and a pulse repetition rate of 1,000 Hz was used to markthe dye-coated sample. The output of the laser was coupled to an opticalfiber, collimated by a lens with a focal length of 150 millimeters andfocused on the sample by a lens with a focal length of 30 millimeters.This assembly was held by a robot arm with the laser focal spot adistance of 6 millimeters away from the sample. The laser was moved at 1millimeter per second and the single pulse energy from the laser waseither 1.0 milliJoules or 0.5 milliJoules at the sample. Magenta lineswere successfully engraved.

Example 4

A solution of 20 weight % solids in THF was prepared containing 400milligrams of Binder-1, 40 milligrams phthalic acid, 40 milligrams ofDye-1, 40 milligrams of magnesium nitrate, and 0.5 milligrams of CuPC.The solution was solvent coated on a glass plate with a #20 Meyer rodand the solvent was allowed to dry. A diode pumped Nd:YAG laser(commercially available from IB Laser Berlin, Germany) at the secondharmonic wavelength (532 nanometers), with pulse duration of 10nanoseconds, and a pulse repetition rate of 1,000 Hz was used to markthe dye-coated sample. The output of the laser was coupled to an opticalfiber, collimated by a lens with a focal length of 100 millimeters toform a beam 18 millimeters in diameter. The laser was then focused onthe sample by a lens with a focal length of 25 millimeters. Thisassembly was placed on an Aerotech (Pittsburgh, Pa.) sample stage. Thelaser spot focused on the sample was around 200 micrometers in diameter.Imaged samples were prepared by varying the laser output and the samplestage translation speed. At an average laser output of 0.535 milliJoulesper pulse and sample stage translation speed of 1.0 millimeter persecond bright cyan colored lines were engraved.

What is claimed is:
 1. An article comprising: a multi-layer constructionwherein at least two layers comprise thermally activatable compositions,the two thermally activatable compositions being the same or different,wherein each thermally activatable composition comprises a non-linearlight to heat converter composition and a color forming compound,wherein the light energy absorption coefficient in the non-linear lightto heat converter compositions is intensity or fluence dependent, whereintensity is energy per unit area per unit time and fluence is energydensity or energy per unit area, and wherein the two thermallyactivatable compositions are selectively activatable by a collimatedlight source.
 2. The article of claim 1 wherein the color formingcompound comprises a leuco dye.
 3. The article of claim 2 wherein theleuco dye comprises an oxazine, a diazine, a thiazine, a fluorescein, arhodamine, a rhodol, a ketazine, a xanthene a barbituric acid leuco dye,a thiobarbituric acid leuco dye or combination thereof.
 4. The articleof claim 1 wherein the thermally activatable composition furthercomprises a thermal acid generator.
 5. The article of claim 4 whereinthe thermal acid generator comprises a diazonium salt with atetrafluoroborate or hexafluorophosphate counterion, a diazonium saltwith a highly fluorinated alkylsulfonyl methide counterion, afluorinated arylsulfonyl methide, a highly fluorinated alkyl sulfonylimide, a diazonium salt with a fluorinated arylsulfonyl imidecounterion, a diazonium salt with a mixed aryl- and alkylsulfonyl imideor methide counterion, a cyclic alcohol with adjacent sulfonate leavinggroups, or combinations thereof.
 6. The article of claim 1 wherein thenon-linear light to heat converter compositions comprise ametallophthalocyanine, a naphthalocyanine, a cyanine, a fullerene, ametal nanoparticle, a metal oxide nanoparticle, a metal clustercompound, a porphyrin, an indanthrone derivative or combination thereof.7. The article of claim 6 wherein the non-linear light to heat convertercompositions comprise copper phthalocyanine, tin phthalocyanine, or acombination thereof.
 8. The article of claim 1 wherein the thermallyactivatable composition further comprises a fixing additive.
 9. Thearticle of claim 8 wherein the fixing additive comprises1-phenyl-3-pyrazolidone, a hydroquinone, a naphthoquinone, ahydroquinone ether, a naphthoquinone ether, a hydronaphthoquinone ether,or mixtures thereof.
 10. The article of claim 1 wherein the multi-layerconstruction comprises at least 3 thermally activatable layers.
 11. Thearticle of claim 10 wherein the thermally activatable layers eachcomprise a different thermally activatable composition, the thermallyactivatable compositions being selected from thermally activatablecompositions which are cyan-forming, magenta-forming, andyellow-forming.
 12. The article of claim 1 wherein the multi-layerconstruction comprises at least 4 thermally activatable layers whereinthe at least 4 thermally activatable layers each comprise a differentthermally activatable composition, the thermally activatablecompositions being selected from thermally activatable compositionswhich are cyan-forming, magenta-forming, yellow-forming andblack-forming.
 13. The article of claim 1 wherein the multi-layerconstruction comprises at least one thermally non-activatable layer. 14.An article comprising: a multi-layer construction wherein a color imageresides within at least two layers and the at least two layers in whichthe color image resides each comprises a reaction product of at leastone thermally activatable composition comprising a non-linear light toheat converter composition and a color forming compound, wherein thelight energy absorption coefficient in the non-linear light to heatconverter compositions is intensity or fluence dependent, whereintensity is energy per unit area per unit time and fluence is energydensity or energy per unit area wherein the two thermally activatablecompositions are the same or different, and wherein the two thermallyactivatable compositions are selectively activatable by a collimatedlight source.
 15. The article of claim 14 wherein the color imagecomprises a multi-layer image.
 16. The article of claim 15 wherein themulti-layer color image comprises at least 3 layers wherein the 3 layerseach comprise a different color selected from the colors cyan, magentaand yellow.
 17. The article of claim 15 wherein the multi-layer colorimage comprises at least 4 layers wherein the 4 layers each comprise adifferent color selected from the colors cyan, magenta, yellow andblack.
 18. The article of claim 14 wherein the color forming compoundcomprises a leuco dye.
 19. The article of claim 18 wherein the leuco dyecomprises an oxazine, a diazine, a thiazine, a fluorescein, a rhodamine,a rhodol, a ketazine, a xanthene a barbituric acid leuco dye, athiobarbituric acid leuco dye or combination thereof.
 20. The article ofclaim 14 wherein the thermally activatable composition further comprisesa thermal acid generator.
 21. The article of claim 14 wherein thenon-linear light to heat converter compositions comprise ametallophthalocyanine, a naphthalocyanine, a cyanine, a fullerene, ametal nanoparticle, a metal oxide nanoparticle, a metal clustercompound, a porphyrin, an indanthrone derivative or combination thereof.22. The article of claim 14 wherein the color image is a fixed image.23. The article of claim 14 wherein the article comprises anidentification document.
 24. The article of claim 23 wherein theidentification document comprises a passport, driver's license, nationalID card, social security card, voter registration and/or identificationcard, birth certificate, police ID card, border crossing card, securityclearance badge, security card, visa, immigration documentation andcard, gun permit, membership card, phone card, stored value card,employee badge, debit card, credit card, and gift certificate and card.